Effective conditioning of this type of seal is essential for safe functioning of many revolving machines, particularly in maintaining a seal required to avoid deformations or damage which might be caused by excessive heating or cooling of the seal.
There are various known solutions for realizing a dry slip ring seal where a revolving shaft passes through a chamber. A labyrinth with reduced gap can be used to confine a non-dangerous gas. Seals with a film of oil or mechanical seals can be used in the case of dangerous gases. The oil circulating on the fixed and mobile ring seals then ensures conditioning either by evacuating the calories produced by its shearing or by adding the calories necessary to maintain a temperature compatible with good functioning.
Such a method of conditioning is not possible in the case of a dry ring seal, which has no such oil circulation.
In heavy duty conditions, as is the case for example for dry ring seals at high speed, high pressure, and/or where a considerable surface area of the revolving parts is in contact with a hot gas, heat generation becomes important and the temperature inside the seal reaches such a value that it can cause destruction of the parts, thus leading to serious failure of a turbo machine.
This risk is particularly sensitive in processes which, for safety reasons, require the mandatory presence of three seal interfaces. The seal comprises three successive internal chambers, from inside the enclosure containing the gas to be confined, out to atmosphere.
The separations between the inside of the enclosure and the first chamber, between two successive chambers, and between the last chamber and atmosphere, are each achieved with the help of a slip ring seal working in conjunction with a fixed ring seal, often referred to as "fixed insert". An auxiliary gas is injected into the intermediate chamber. This auxiliary gas passes by leakage into the first and third chamber, from each of which it is extracted. Heat is evacuated partly by the leakages of gas into the seal interfaces.
It should however be noted that the gas extracted from the first chamber is polluted by the gas to be confined and must in some cases be treated. A known solution for evacuating calories from such a seal consists of increasing the amount of gas leaking through this seal.
But this may prove dangerous, for the following reasons:
leakage of the confined gas to enable evacuation of the heat may lead to operating safety problems, PA1 too great an axial gap between the fixed and the slip ring seals reduces the axial resistance of the gas film circulating radially within this space. Under the effect of various mechanical and aerodynamic stresses, spurious contacts may occur as a result between a fixed ring and a slip ring. Such contacts risk leading to an ultimate loss of impermeability. The frequently encountered problem of cooling, although important, is not the only problem that needs to be taken into account. Indeed, in some installations, a dry slip ring seal is placed in a very cold ambiance, and a hot gas is injected into the seal interface. In this case, if heat generation is insufficient, there results a cooling of the gas which may cause a liquid (condensation . . . ) to form, thereby destroying the seal. To avoid the gas cooling, one solution would be to increase the flow, but in this case would lead to the disadvantages mentioned earlier.